docs: update plugin design doc

[FinleyGe]

• chore: update docker-compose.yamls
• docs(plugin): add design document for plugin

[gru-agent]

TestGru Assignment

Summary

Link	CommitId	Status	Reason
Detail	dbcc94a	🚫 Skipped	No files need to be tested {"deploy/docker/docker-compose-milvus.yml":"File path does not match include patterns.","deploy/docker/docker-compose-oceanbase/docker-compose.yml":"File path does not match include patterns.","deploy/docker/docker-compose-pgvector.yml":"File path does not match include patterns.","deploy/docker/docker-compose-zilliz.yml":"File path does not match include patterns.","docSite/content/zh-cn/docs/guide/plugins/design_plugin.md":"File path does not match include patterns."}

History Assignment

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